Update opm-flowdiagnostics system probe for API change

This commit brings the system probe for module

    opm-flowdiagnostics

up-to-date with recent changes in the public interface of the
module's main entry point.  Commit OPM/opm-flowdiagnostics@8456e81
removed the nested type

    Opm::FlowDiagnostics::Toolbox::PoreVolume

so we must drop all references to this type.

cmake/Modules/Findopm-flowdiagnostics.cmake

@@ -40,7 +40,7 @@ int main()
 {
     using FDT = Opm::FlowDiagnostics::Toolbox;
 
-    const auto pv = FDT::PoreVolume{ std::vector<double>(10, 0.3) };
+    const auto pv = std::vector<double>(10, 0.3);
 }
 "
   # config variables